Keywords:

Abstract:

Planar images of laser-induced fluorescence from OH
radicals and elastic scattering from soot particles are
presented in time-varying laminar CH4/air diffusion
flames burning in a co-flowing, axisymmetric
configuration at atmospheric pressure. Acoustic forcing
is used to phase lock the periodic flame flicker to the
pulsed laser system operating at 10.13 Hz. For
conditions where the tip of the flame is clipped, the
intensity of the light scattered by the soot particles
increases dramatically (by more than a factor of 7 for
the maximum signals at a point) compared to a
steady-state, laminar flame with the same mean fuel flow
velocity. Comparison of the scattering signals
integrated along the flame radius is carried out in the
steady-state and time-varying flames as a function of
height above the burner. Time-varying flames exhibit a
larger range of combustion conditions than observed in
corresponding steady-state flames, including different
residence times, temperature histories, local
stoichiometries, and strain and scalar dissipation
rates. Thus, their investigation promises to yield new
insights into a wide variety of chemistry-flowfield
interactions which are prominent in turbulent
combustion.

The National Institute of Standards and Technology (NIST) is an agency of the U.S. Commerce Department.